1. Field of the Invention
The invention relates to a circuit for detecting a level or a level variation of an input direct voltage, comprising an input terminal, to which the input direct voltage can be applied, and comprising a comparator having a first input, a second input and an output, which first input is connected to the input terminal, which second input is arranged to receive a reference signal, and on which output a comparator signal is available, and comprising a control information generation device by means of which control information corresponding to the comparator signal of the comparator can be generated, and comprising a controllable pulse generator for generating pulses which are available on an output of the pulse generator and which are variable with respect to their charge quantity by means of the control information which corresponds to the comparator signal and which can be applied to the pulse generator by the control information generation device, and comprising an integrator having an input connected to the output of the pulse generator and having an output connected to the second input of the comparator and provided to form the reference signal with the aid of the pulses of the pulse generator, the reference signal having a d.c. component corresponding to the instantaneous charge quantity of the pulses of the pulse generator, and having an a.c. component superposed on the d.c. component.
2. Description of Related Art
Such a circuit of the type defined in the opening paragraph is, for example, known form EP 0 381 217 A2. In said known circuit a level or a level variation of an input direct voltage which can vary is effected merely on the basis of the value of the charge quantity of the pulses from the pulse generator, which charge quantity is determined by the duration of these pulses, as a result of which only a comparatively low resolution within a given range of levels or level variations to be detected is attainable, so that only a comparatively coarse detection of a level or level variation is possible. Moreover, the pulse generator in the known circuit does not operate continuously but intermittently during cyclically succeeding operating periods which are spaced from one another by pause intervals, which may give rise to undesirable transient effects at the beginning of each operating period. Furthermore, it is to be noted that in the known circuit a capacitor in the integrator is charged and subsequently discharged substantially completely in cyclic succession in dependence on the pulses of the pulse generator, so that residual charges left in the capacitor after discharging can have an adverse effect.